Nuclear steam supply system

ABSTRACT

A nuclear steam supply system includes an elongated reactor vessel having an internal cavity with a central axis, a reactor core having nuclear fuel disposed within the internal cavity, and a steam generating vessel having at least one heat exchanger section, the steam generating vessel being fluidicly coupled to the reactor vessel. The reactor vessel includes a shell having an upper flange portion and a head having a head flange portion. The upper flange portion is coupled to the head flange portion, wherein the upper flange portion extends into the internal cavity, and the head flange portion extends outward from the internal cavity.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/859,934 filed Jan. 2, 2018, which is a continuation of U.S.patent application Ser. No. 14/398,946 filed Nov. 4, 2014, which is anational stage application under 35 U.S.C. § 371 to internationalapplication No. PCT/US2013/039743 filed May 6, 2013, which claimspriority to U.S. Provisional Patent Application No. 61/642,614, filedMay 4, 2012; the disclosures of which are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The field of the present invention relates to nuclear steam supplysystems, and more particularly to a steam supply system for smallmodular reactors.

BACKGROUND OF THE INVENTION

Pressurized water reactors (PWRs) for nuclear power generationfacilities utilize both pumped and natural circulation of the primarycoolant to both cool the reactor core and heat the secondary coolant toproduce steam which may be working fluid for a Rankine power generationcycle. The existing natural circulation PWRs suffer from the drawbackthat the heat exchange equipment is integrated with and located withinthe reactor pressure vessel. Such an arrangement not only makes the heatexchange equipment difficult to repair and/or service, but also subjectsthe equipment to corrosive conditions and results in increasedcomplexity and a potential increase in the number of penetrations intothe reactor pressure vessel. In addition, locating the heat exchangeequipment within the reactor pressure vessel creates problems withrespect to radiation levels encountered for crews to repair the heatexchange equipment in proximity to the radioactively hot components ofthe reactor vessel. The general view has also been that the heatexchangers should be located in the reactor vessel to achieve naturalcirculation in those systems which may utilize this type of flowcirculation.

The reduction of vulnerabilities within nuclear power generationfacilities is always an ongoing issue. For example, large pipes are seenas creating the potential for a “large break” Loss of Coolant Accident(LOCA) event, and thus it is desirable to remove large pipes wherepossible.

SUMMARY OF THE INVENTION

A nuclear reactor vessel includes a shell and a head affixed to theupper end of the shell. The shell has an internal cavity with a centralaxis and an upper flange portion, wherein the internal cavity isconfigured to receive a reactor core. The head has a head flangeportion, with the upper annular flange portion is coupled to the headannular flange portion, and the flanges are configured to minimizeoutward extension from the cavity while still providing desired leakprotection at the interface between the shell and the head.

In a first separate aspect of the present invention, the upper flangeportion of the shell is annular and extends into the internal cavity,and the head flange portion of the head is also annular and extendsoutward from the internal cavity.

In a second separate aspect of the present invention, a reactor coreincluding nuclear fuel is disposed within the internal cavity of thenuclear reactor vessel, and a steam generating vessel including at leastone heat exchanger section is fluidicly coupled to the reactor vessel.The upper flange portion of the shell extends into the internal cavity,and the head flange portion of the head extends outward from theinternal cavity.

In a third separate aspect of the present invention, a reactor coreincluding nuclear fuel is disposed within the internal cavity of thenuclear reactor vessel, and a steam generating vessel including at leastone heat exchanger section is fluidicly coupled to the reactor vessel.The upper flange portion of the shell extends into the internal cavity,and the head flange portion of the head extends outward from theinternal cavity. An inner surface of the first head portion is disposedcloser to the central axis than an inner surface of the first shellportion along respective parallel radial lines extending from thecentral axis.

In a fourth separate aspect of the present invention, a method forgenerating steam utilizes the nuclear reactor vessel. The reactor vesselis capped with a head, and a reactor core is disposed within the reactorvessel. The upper flange portion extends into the internal cavity, andthe head flange portion extends outward from the internal cavity. Aliquid primary coolant is heated in the nuclear reactor core, and theheated primary coolant is discharged from a top portion of the reactorvessel into a steam generating vessel. The primary coolant is flowedthrough the reactor vessel and steam generating vessel in a closedcirculation loop.

In a fifth separate aspect of the present invention, one or more of thepreceding separate aspects may be employed in combination.

Advantages of the improvements will be apparent from the drawings andthe description of the embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe exemplary embodiments, will be better understood when read inconjunction with the appended drawings. It should be understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown in the following figures:

FIG. 1 is cross section of a containment vessel including a nuclearsteam supply system;

FIG. 2 is a schematic flow diagram of the nuclear steam supply system ofFIG. 1 showing the flow path or circulation flow loop of primary coolantrepresented by the flow arrows;

FIG. 3A is an elevation cross-sectional view of the reactor vessel ofFIG. 1;

FIG. 3B is a detailed cross-sectional view of the reactor vessel of FIG.3A;

FIG. 4 is an elevation cross-sectional view of the bottom portion of thesteam generating vessel reactor vessel of FIG. 1 showing the preheatersection and steam generator section;

FIG. 5 is an elevation cross-sectional view of the top portion of thesteam generating vessel of FIG. 1 showing the superheater section andpressurizer mounted atop the steam generating vessel; and

FIG. 6 is an exploded perspective view of the fuel cartridge positionedin the reactor vessel of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, any reference to direction or orientation is merelyintended for convenience of description and is not intended in any wayto limit the scope of the present invention. Relative terms such as“lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,”“down,” “left,” “right,” “top” and “bottom” as well as derivativesthereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description only and do not require that the apparatus be constructedor operated in a particular orientation unless explicitly indicated assuch. Terms such as “attached,” “affixed,” “connected,” “coupled,”“interconnected,” and similar refer to a relationship wherein structuresare secured or attached to one another either directly or indirectlythrough intervening structures, as well as both movable or rigidattachments or relationships, unless expressly described otherwise.Moreover, the features and benefits of the invention are illustrated byreference to the preferred embodiments. Accordingly, the inventionexpressly should not be limited to such preferred embodimentsillustrating some possible non-limiting combinations of features thatmay exist alone or in other combinations of features; the scope of theinvention being defined by the claims appended hereto.

Referring to FIGS. 1-6, a steam supply system for a nuclear pressurizedwater reactor (PWR) according to the present disclosure is shown. Fromthe thermal-hydraulic standpoint, the system includes a steam generatorassembly 100 generally including a reactor vessel 200 and a steamgenerating vessel 300 fluidly coupled to the reactor vessel. The steamgenerating vessel and reactor vessel are vertically elongated andseparate components which hydraulically are closely coupled, butdiscrete vessels in themselves that are thermally isolated except forthe exchange of primary loop coolant (i.e. reactor coolant) flowingbetween the vessels. As further described herein, the steam generatingvessel 300 in one embodiment includes a preheater 320, main steamgenerator 330, and a superheater 350 which converts a fluid such aswater flowing in a secondary coolant loop from a liquid entering thesteam generating vessel 300 at an inlet 301 to superheated steam leavingthe steam generating vessel at an outlet 302. The secondary coolant loopwater may be a Rankine cycle fluid used to drive a turbine-generator setfor producing electric power in some embodiments.

The steam generating vessel 300 further includes a pressurizer 380 whichmaintains a predetermined pressure of the primary coolant fluid. Thepressurizer is a pressure vessel mounted atop the steam generatingvessel 300 and engineered to maintain a liquid/gas interface (i.e.primary coolant water/inert gas) that operates to enable control of theprimary coolant pressure in the steam generator. In one embodiment, asshown, the pressurizer 380 may be mounted directly on top of the steamgenerating vessel 300 and forms an integral unitary structural part ofthe vessel to hydraulically close the vessel at the top end. Theassemblage of the foregoing three heat exchangers and the pressurizermay be referred to as a “stack.”

Referring to FIG. 1, the reactor vessel 200 and the steam generatingvessel 300 are housed in a steam generator containment vessel 110. Thecontainment vessel 110 may be formed of a suitable shop-fabricated steelcomprised of a top 111, a bottom 112, and a cylindrical sidewall 113extending therebetween. In some embodiments, portions of the containmentvessel which are located above ground level may be made of ductileribbed steel to help withstand aircraft impact. A missile shield 117which is spaced above the top 111 of the containment vessel 110 may beprovided as part of the containment vessel or a separate containmentenclosure structure (not shown) which encloses the containment vessel110. A horizontal partition wall 114 divides the containment vessel intoan upper portion 114 a and a lower portion 114 b. The partition wall 114defines a floor in the containment vessel. In one embodiment, a majorityof the reactor vessel 200 may be disposed in the lower portion 114 b andthe steam generating vessel 300 may be disposed in the upper portion 114a as shown.

In various embodiments, the containment vessel 110 may be mounted aboveground, partially below ground, or completely below ground. In certainembodiments, the containment vessel 110 may be positioned so that atleast part or all of the lower portion 114 b that contains the nuclearfuel reactor core (e.g., a fuel cartridge 230) is located below groundlevel. In one embodiment, the entire reactor vessel 200 and a portion ofthe steam generating vessel 300 are located entirely below ground levelfor maximum security. The cylindrical shell or sidewall 113 of thecontainment vessel 110 may be horizontally split into an upper sectionand a lower section, which are joined together by a circumferentialwelded or bolted flanged joint 119 as shown in FIG. 1 to provide ademarcation for portions of the containment vessel which are locatedabove and below ground level. In other embodiments, the upper and lowersections may be welded together without use of a flange.

In one embodiment, for example without limitation, the containmentvessel 110 may have a representative height of approximately 200 feet ormore for a 160 MW (megawatt) modular nuclear electric generationfacility. A non-limiting representative diameter for this powergeneration facility is about 45 feet. Any suitable height and diameterfor the containment vessel may be provided depending on system componentconfiguration and dimensions.

The containment vessel 110 further includes a wet reactor well 115defined in one embodiment by a cylindrical circumscribing walledenclosure 116 which is flooded with water to provide enhanced radiationshielding and a back-up reserve of readily accessible coolant for thereactor core. In one embodiment, the walled enclosure 116 may be formedof stainless steel cylindrical walls which extend circumferentiallyaround the reactor vessel 200 as shown. Other suitable materials may beused to construct the enclosure 116. The wet reactor well 115 isdisposed in the lower portion 114 b of the containment vessel 110. Thelower portion 114 b may further include a flooded (i.e. water) used fuelpool 118 adjacent to the enclosure 116. In one embodiment, as shown inFIG. 1, both the used fuel pool 118 and the walled enclosure 116 aredisposed below the horizontal partition wall 114 as shown in FIG. 1.

In one embodiment, as shown in FIG. 1, the walled enclosure 116 mayextend above the partition wall 114 and the inlet/outlet nozzleconnection between the reactor and steam generating vessels may be madeby a penetration through the walled enclosure.

As further shown in FIG. 1, both the reactor vessel 200 and the steamgenerating vessel 300 preferably may be vertically oriented as shown toreduce the footprint and diameter of the containment vessel 110. Thecontainment vessel 110 has a diameter large enough to house both thereactor vessel, steam generating vessel, and any other appurtenances.The containment vessel 110 preferably has a height large enough tocompletely house the reactor vessel and steam generating vessel toprovide a fully contained steam generator with exception of the waterand steam inlet and outlet penetrations for second coolant loop fluidflow associated with the Rankine cycle for driving the turbine-generatorset for producing electric power.

FIG. 2 shows the flow or circulation of primary coolant (e.g. water) inthe primary coolant loop. In one embodiment, the primary coolant flow isgravity-driven relying on the change in temperature and correspondingdensity of the coolant as it is heated in the reactor vessel 200, andthen cooled in the steam generating vessel 300 as heat is transferred tothe secondary coolant loop of the Rankine cycle which drives theturbine-generator (T-G) set. The pressure head created by the changingdifferent densities of the coolant (i.e. hot—lower density andcold—higher density) induces flow or circulation through the reactorvessel-steam generating vessel system as shown by the directional flowarrows. Advantage, the gravity-driven primary coolant circulationrequires no coolant pumps or machinery thereby resulting in cost(capital, operating, and maintenance) savings, reduced system powerconsumption thereby increasing energy conversion efficiency of the PWRsystem, in addition to other advantages as described herein.

The reactor vessel 200 may be similar to the reactor vessel withgravity-driven circulation system disclosed in commonly-owned U.S.patent application Ser. No. 13/577,163 filed Aug. 3, 2012, thedisclosure of which is incorporated herein by reference in its entirety.

Referring to FIGS. 3A and 3B, the reactor vessel 200 in one embodimentis an ASME code Section III, Class 1 thick-walled cylindrical pressurevessel includes a cylindrical sidewall shell 201, an integrally weldedhemispherical bottom head 203 and, a removable hemispherical top head202. The shell 201 primarily defines an internal cavity 208 configuredfor holding the reactor core, reactor shroud, and other appurtenances asdescribed herein. In one embodiment, the upper extremity of the reactorvessel shell 201 is equipped with a tapered hub flange 204 (also knownas “welding neck” flange in the art) which is bolted to a similar flange205 welded to the top head 202. Commonly-owned PCT patent applicationNo. PCT/US2013/0038289, filed Apr. 25, 2013, the disclosure of which isincorporated herein by reference in its entirety, discloses knownprior-art for the design and coupling of the top head to the shell usingtwo flanges.

Each flange 204, 205 may be annular, so that each extends completelyaround the shell 201 and the head 202, respectively. Each flange mayalso be integrally formed as part of the shell 201 and the top head 202.The flange 204 extends into and toward the central axis 209 of thecavity 208, with the flange 204 forming at about the point where thesidewalls of the shell 201 begin to widen. In the case that the flange204 is annular, it extends radially into the cavity around the entirecavity, and similarly, in the case that the flange 205 is annular, itextends radially outward from the cavity all around. As can be seen inthe embodiment depicted, the inner sidewall surfaces 209 of the shell201, excluding the flange 204, are defined by a first inner radius,measured from the central axis 210 of the cavity 208, and the innersidewall surfaces 211 of the flange 204 are defined by a second innerradius, with the second inner radius being smaller than the first innerradius. The outward-extending head flange 205, which is formed at aboutthe point where the sidewalls of the top head 202 begin to widen, has aninner surface 212 that may be at about the same distance from thecentral axis 210 as the inner sidewall surfaces 211 of the shell flange204, as can be seen by line A. Thus, the inner surfaces 211, 212 of thetwo flanges 204, 205 have about the same radius from the central axis210, and the inner surfaces 212 of the flange 205 have a smaller radiusthan the inner sidewall surfaces 209 of the shell 201. Also, the outersurfaces 213 of the hemispherical wall of the top head 202, at a pointjust above the flange 205, may be at about the same distance from, oreven closer to, the central axis 210 as the inner sidewall surfaces 211of the shell flange 204, as can be seen by line B. This results in theradius of the outer surfaces 213 having about the same radius from thecentral axis 210 as the inner sidewall surfaces 211, although the outersurfaces 213 could also have a radius less than that of the innersidewall surfaces 211. So that the coupled flanges 204, 205 may eachstill serve as a “welding neck” flange, the outer surfaces 214, 215 ofeach flange may be at about the same distance from the central axis 210,as can be seen by line C. All distance and measurement comparisonsbetween the shell 201 and the top head 210 are being made along parallelradial lines having the central axis 210 as a center.

The top head 202 may be fastened to the shell 201 by coupling theflanges 204, 205 via a set of alloy bolts 216, which are pre-tensionedto establish a high integrity double gasket seal under all operationmodes. The bolted connection of the top head 202 provides ready accessto the reactor vessel internals such as the reactor core. Thecenterline, line D, of the bolts 216 may be at a distance greater thanthe internal surfaces 209 of the shell 201, but at a lesser distancethan the outer surfaces 214 of the shell flange 204.

Two concentric self-energizing gaskets 206 are placed in a pair ofannular grooves 218, the grooves being formed in both flanges 204, 205,between the bolts 216 and the inner surfaces 211, 212, and compressedbetween the interfacing surfaces of two flanges 204, 205, when coupledtogether, to provide leak tightness of the reactor vessel 200 at theconnection between the top head 202 and the shell 201. The leaktightness under operating conditions is assured by an axisymmetricheating of the flanged joint that is provided by the fluid flowarrangement of the primary coolant in the system, as further describedherein. The top head 202 contains the vertical penetrations 207 forinsertion of the control rods and further may serve as a base formounting the associated control rod drives, both of which are notdepicted but well known in the art without further elaboration.

With continuing reference to FIG. 3A, the reactor vessel 200 includes acylindrical reactor shroud 220 which contains the reactor core definedby a fuel cartridge 230. The reactor shroud 220 transversely divides theshell portion of the reactor vessel into two concentrically arrangedspaces: (1) an outer annulus 221 defining an annular downcomer 222 forprimary coolant entering the reactor vessel which is formed between theouter surface of the reactor shroud and the inner surface of the shell201; and (2) a passageway 223 defining a riser column 224 for theprimary coolant leaving the reactor vessel heated by fission in thereactor core. The reactor shroud 220 is elongated and extends in anaxial direction along vertical axis VA1 of the reactor vessel whichdefines a height and includes an open bottom 225 and a closed top 226.In one embodiment, the top 226 may be closed by a top flow isolationplate 227 which directs primary coolant flowing up the riser column 224to the steam generating vessel 300, as further described herein. In oneembodiment, the bottom 225 of the reactor shroud 220 is verticallyspaced apart by a distance from the bottom head 203 of the reactorvessel 200 and defines a bottom flow plenum 228. The bottom flow plenum228 collects primary coolant from the annular downcomer 222 and directsthe coolant flow into the inlet of the riser column 224 formed by theopen bottom 225 of the reactor shroud 220 (see, e.g. FIG. 2).

Both the fuel cartridge 230 and the reactor shroud 220 are supported bya core support structure (“CSS”), which in one embodiment includes aplurality of lateral support members 250 that span between and areattached to the reactor shroud and the shell 201 of the reactor vessel200. A suitable number of supports members space both circumferentiallyand vertically apart are provided as needed to support the combinedweight of the fuel cartridge 230 and the reactor shroud 220. In oneembodiment, the bottom of the reactor shroud 220 is not attached to thereactor vessel 200 to allow the shroud to grow thermally in a verticalaxial direction (i.e. parallel to vertical axis VA1) without undueconstraint.

The reactor shroud 220 is a double-walled cylinder in one embodimentwhich may be made of a corrosion resistant material, such as withoutlimitation stainless steel. This double-wall construction of the reactorshroud 220 forms an insulated structure designed to retard the flow ofheat across it and forms a smooth vertical riser column 224 for upwardflow of the primary coolant (i.e. water) heated by the fission in thefuel cartridge 230 (“core”), which is preferably located at the bottomextremity of the shroud in one embodiment as shown in FIGS. 1-3. Thevertical space above the fuel cartridge 230 in the reactor shroud 220may also contain interconnected control rod segments along with a set of“non-segmental baffles” that serve to protect them from flow inducedvibration during reactor operations. The reactor shroud 220 is laterallysupported by the reactor vessel by support members 250 to prevent damagefrom mechanical vibrations that may induce failure from metal fatigue.

The fuel cartridge 230 in one embodiment is a unitary autonomousstructure containing upright fuel assemblies, and is situated in aregion of the reactor vessel 200 that is spaced above the bottom head203 so that a relatively deep plenum of water lies underneath the fuelcartridge. The fuel cartridge 230 is insulated by the reactor shroud 220so that a majority of the heat generated by the fission reaction in thenuclear fuel core is used in heating the primary coolant flowing throughthe fuel cartridge and adjoining upper portions of the riser column 224.The fuel cartridge 230 is an open cylindrical structure includingcylindrically shaped sidewalls 231, an open top 233, and an open bottom234 to allow the primary coolant to flow upward completely through thecartridge (see directional flow arrows). In one embodiment, thesidewalls 231 may be formed by multiple arcuate segments of reflectorswhich are joined together by suitable means. The open interior of thefuel cartridge 230 is filled with a support grid 232 for holding thenuclear fuel rods and for insertion of control rods into the core tocontrol the fission reaction as needed.

Briefly, in operation, the hot reactor primary coolant exits the reactorvessel 200 through a low flow resistance outlet nozzle 270 to be cooledin the adjacent steam generating vessel 300, as shown in FIGS. 2 and 3.The cooled reactor primary coolant leaves the steam generating vessel300 and enters the reactor vessel 200 through the inlet nozzle 271. Theinternal plumbing and arrangement in the reactor vessel directs thecooled reactor coolant down through to the annular downcomer 222. Theheight of the reactor vessel 200 is preferably selected to support anadequate level of turbulence in the recirculating reactor primarycoolant by virtue of the density differences in the hot and cold watercolumns which is commonly known as the thermo-siphon action (densitydifference driven flow) actuated by gravity. In one embodiment, thecirculation of the reactor primary coolant is driven by over 8 psipressure generated by the thermo-siphon action, which has beendetermined to ensure (with adequate margin) a thoroughly turbulent flowand stable hydraulic performance.

Referring to FIGS. 1 and 3, the top of the reactor vessel shell 201 iswelded to a massive upper support forging which may be referred to as areactor support flange 280. The support flange 280 supports the weightof the reactor vessel 200 and internal components above the wet reactorwell 115. In one embodiment, the support flange is structurallystiffened and reinforced by a plurality of lugs 281 which are spacedcircumferentially apart around the reactor vessel and welded to both thereactor vessel and flange, as shown. Support flange contacts and engagesthe horizontal partition wall 114, which transfers the dead weight ofthe reactor vessel 200 to the containment vessel 110. The reactorvessel's radial and axial thermal expansion (i.e. a majority of growthbeing primarily downwards from the horizontal partition wall 114) as thereactor heats up during operation is unconstrained. However, the portionof the containment vessel 110 which projects above the partition wall114 is free to grow upwards in unison with the upwards growth of thesteam generating vessel 30 to minimize axial differential expansionbetween the steam generating vessel and reactor vessel. Because thereactor vessel and steam generating vessel are configured and structuredto thermally grow in height at substantially the same rate when heated,this arrangement helps minimize potential thermal expansions stress inthe primary coolant fluid coupling 273 between the reactor vessel andsteam generating vessel.

The support flange 280 is spaced vertically downwards on the reactorvessel shell 201 by a distance from the top head 202 of the reactorvessel 200 sufficient to allow a fluid connection to be made to thesteam generating vessel 300 which is above the partition wall 114, asshown in FIGS. 1 and 2. When the reactor vessel 200 is mounted insidethe containment vessel 110, the top head 202 of the reactor vessel andthe primary coolant fluid coupling 273 (collectively formed by combinedthe inlet-outlet flow nozzle 270/271 and the inlet-outlet flow nozzle371/370 of the steam generating vessel 300, shown in FIG. 4) are locatedabove the reactor well 115. This provides a location for connection tothe steam generator headers and for the engineered safety systems (e.g.control rods, etc.) to deal with various postulated accident scenarios.A majority of the reactor vessel shell 201, however, may be disposedbelow the partition wall 114 and immersed in the wet reactor well 115 asshown in FIG. 1.

The bottom region of the reactor vessel 200 is restrained by a lateralseismic restraint system 260 (shown schematically in FIG. 1) that spansthe space between the reactor shell 201 and the reactor well 115 insidesurface of the cylindrical enclosure 116 to withstand seismic events.The seismic restraint design is configured to allow for free axial (i.e.longitudinal along vertical axis VA1) and diametrical thermal expansionof the reactor vessel 200. The reactor well 115 is flooded during poweroperations to provide defense-in-depth against a (hypothetical,non-mechanistic) accident that is assumed to produce a rapid rise in theenthalpy of the reactor's contents. Because the reactor is designed toprevent loss of core water by leaks or breaks and the reactor well isflooded, burn-through of the reactor vessel by molten fuel (corium) isnot likely.

Referring to FIGS. 3 and 4, the combined inlet-outlet flow nozzle270/271 has two concentric hollow forgings including an outer inletnozzle 270 and an inner outlet nozzle 271. The outlet nozzle 271 has oneend welded to the reactor shroud 220 (internal to the reactor vesselshell 201) and an opposite end welded to the inlet nozzle 371 of thesteam generating vessel 300. The inlet nozzle 270 has one end welded tothe reactor vessel shell 201 and an opposite end welded to the outletnozzle 370 of the steam generating vessel 300. The flow isolation plate227 helps ensure that the hot primary coolant water exiting the reactorvessel cannot flow back into the annulus 221. In the present embodiment,the outlet nozzle 271 of the reactor vessel and the inlet nozzle 371 ofthe steam generating vessel each have a smaller diameter than the inletnozzle 270 of the reactor vessel and the outlet nozzle 270 of the steamgenerating vessel. The combined inlet-outlet flow nozzle 270/271 islocated above the partition wall 114 of the containment vessel 110. Theinlet nozzle 371 and the outlet nozzle 370 of the steam generatingvessel 300 collectively define a mating concentrically arranged combinedinlet/outlet nozzle 371/370 for the steam generating vessel.

In order to avoid long loops of large piping in the reactor primarycoolant system which creates the potential for a “large break” LOCAevent, both the combined inlet-outlet flow nozzle 270/271 of the reactorvessel 200 and the combined inlet/outlet nozzle 371/370 for the steamgenerating vessel are intentionally very closely coupled to the shellsof their respective vessels having a minimal radial projection beyondthe shells. The design of the top of the reactor vessel, with theflanged connection between the head and the shell of the reactor vessel,helps to minimize this radial projection beyond the shell. This isaccomplished by reducing the extent to which the flanges extend out fromthe shell, as compared to the prior art. In addition, cost advantagesmay be realized in having the inlet-outlet flow nozzle 270/271shortened, in that different manufacturing techniques may be used tocreate the shorter inlet-outlet flow nozzle 270/271 as compared to if alonger flow nozzle is required.

This permits the reactor vessel 200 to be directly coupled to the steamgenerating vessel 300 via the inlet/outlet nozzles as shown in FIGS. 1and 2. As shown in FIG. 3A, the combined inlet-outlet flow nozzle270/271 of the reactor vessel preferably protrudes radially beyond theshell 201 by a distance that is no more than the radial projection ofthe support flange 280. The total length of the inlet/outlet nozzleconnection between the reactor vessel 200 and steam generating vessel300 in certain embodiment is less than or equal to the diameter of thereactor vessel 200, and/or the steam generating vessel 300 to eliminatelong runs of large coolant piping between the reactor and steamgenerating vessels. In one embodiment, the nozzle connections betweenthe reactor vessel 200 and the steam generating vessel 300 is straightwithout any elbows or bends.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques. It is tobe understood that other embodiments may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Thus, the spirit and scope of the inventionshould be construed broadly as set forth in the appended claims.

What is claimed is:
 1. A nuclear steam supply system comprising: avertically elongated reactor vessel comprising an internal cavity with acentral axis and a primary coolant flow nozzle extending outwards fromthe reactor vessel; a reactor core comprising nuclear fuel disposedwithin the internal cavity; a steam generating vessel comprising atleast one heat exchanger section, the steam generating vessel beingfluidly coupled to the reactor vessel; wherein the reactor vesselcomprises a cylindrical shell having an annular upper flange portion,and a head having an annular head flange portion, the upper flangeportion being removably coupled to the head flange portion, and whereinthe upper flange portion extends inwards into the internal cavity, andthe head flange portion extends outward from the internal cavity.
 2. Thesystem according to claim 1 wherein the shell includes a first shellportion having a first inner radius, and the head includes a first headportion having a second inner radius, which is smaller than the firstinner radius.
 3. The system according to claim 2, wherein the upperflange portion of the shell that extends into the central cavity definesa third inner radius which is about the same as the second inner radiusof the first head portion and smaller than the first inner radius of thefirst shell portion.
 4. The system according to claim 1 wherein theshell includes a first shell portion having an inner radius, and thehead includes a first head portion above the head flange portion havingan outer radius, which is smaller than or equal to the inner radius. 5.The system according to claim 1 wherein the shell includes a first shellportion having a first inner surface, and the head includes a first headportion above the head flange portion having a second inner surface, andwherein the second inner surface is closer to the central axis than thefirst inner surface along respective parallel radial lines extendingfrom the central axis.
 6. The system according to claim 1 wherein theshell includes a first shell portion below the upper flange portion ofthe shell, the first shell portion having a first inner surface and theupper flange portion of the shell having a second inner surface, andwherein the second inner surface is closer to the central axis than thefirst inner surface along respective parallel radial lines extendingfrom the central axis.
 7. The system according to claim 6, wherein theprimary coolant flow nozzle is disposed on the first shell portion. 8.The system according to claim 7, wherein the primary coolant flow nozzleis formed as an integral forging with the shell of the reactor vessel.9. The system according to claim 7, further comprising an annularsupport flange attached to the first shell portion of the reactorvessel, the support flange configured for mounting on a horizontalsurface to support a weight of the reactor vessel.
 10. The systemaccording to claim 9, wherein the support flange does not protruderadially outwards from the reactor vessel farther than the primarycoolant flow nozzle.
 11. The system according to claim 1 wherein theshell includes a first shell portion having an inner surface, and thehead includes a first head portion above the head flange portion havingan outer surface, and wherein the outer surface is at least as close tothe central axis as the inner surface along respective parallel radiallines extending from the central axis.
 12. The system according to claim1 wherein the shell includes a first shell portion having a first outersurface, the head includes a first head portion having a second outersurface, and a plurality of fasteners secure the upper flange portion tothe head flange portion, and wherein the fasteners are each disposedfurther from the central axis than the second outer surface and nearerthe central axis than the first outer surface.
 13. The system accordingto claim 12, further comprising a plurality of circumferentiallyarranged longitudinal bores formed in both the upper flange portion ofthe shell and the head flange portion of the head that receive thefasteners, wherein a first distance between each bore and the centralaxis of the internal cavity of the reactor vessel is less than a seconddistance between the central axis and an outer surface of the firstshell portion of the reactor vessel.
 14. The system according to claim13, wherein the longitudinal bores in the upper flange portion of theshell extend partially into the upper flange portion and thelongitudinal bores in the head flange portion of the head extendcompletely through the head flange portion.
 15. The system according toclaim 13, wherein the fasteners are threaded fasteners.
 16. A nuclearsteam supply system comprising: a vertically elongated reactor vesselcomprising an internal cavity with a central axis and a primary coolantflow nozzle extending outwards from the reactor vessel; a reactor corecomprising nuclear fuel disposed within the internal cavity; a steamgenerating vessel comprising at least one heat exchanger section, thesteam generating vessel being fluidly coupled to the reactor vessel; thereactor vessel further comprising a cylindrical shell having a firstportion, an annular upper flange portion, and a removable head having anannular head flange portion coupled to the upper flange portion, whereinthe upper flange portion extends inwards into the internal cavity, andthe head flange portion extends outward from the internal cavity; aplurality of circumferentially arranged longitudinal bores formed in theupper flange portion of the shell and the head flange portion of thehead, the upper flange portion being coupled to the head flange portionby a plurality of threaded fasteners received in the bores; wherein afirst distance between each bore and central axis of the internal cavityof the reactor vessel is less than a second distance between the centralaxis and an outer surface of the first shell portion of the reactorvessel.
 17. The system according to claim 16 wherein the first shellportion of the shell has first inner radius, and the head flange portionof the head has a second inner radius, which is smaller than the firstinner radius.
 18. The system according to claim 17, wherein the upperflange portion of the shell that extends into the central cavity definesa reduce diameter portion having third inner radius which issubstantially the same as the second inner radius of the head flangeportion and smaller than the first inner radius of the first shellportion.
 19. A nuclear steam supply system comprising: a verticallyelongated reactor vessel comprising an internal cavity, a central axis,and a primary coolant flow nozzle extending outwards from the reactorvessel, the primary coolant flow nozzle formed as an integral part of aforging with the reactor vessel; a reactor core comprising nuclear fueldisposed within the internal cavity; an annular support flange attachedto the reactor vessel and configured for mounting on a horizontalsurface to support a weight of the reactor vessel; a verticallyelongated steam generating vessel comprising at least one heat exchangersection, the steam generating vessel being fluidly coupled to thereactor vessel; the reactor vessel further comprising a cylindricalshell having a first portion, an annular upper flange portion, and aremovable head having an annular head flange portion coupled to theupper flange portion, wherein the upper flange portion extends inwardsinto the internal cavity, and the head flange portion extends outwardfrom the internal cavity; a plurality of circumferentially arrangedlongitudinal bores formed in the upper flange portion of the shell andthe head flange portion of the head, the upper flange portion beingcoupled to the head flange portion by a plurality of threaded fastenersreceived in the bores; wherein a first distance between each bore andcentral axis of the internal cavity of the reactor vessel is less than asecond distance between the central axis and an outer surface of thefirst shell portion of the reactor vessel.
 20. The system according toclaim 19 wherein the first shell portion of the shell has first innerradius, the head flange portion of the head has a second inner radiussmaller than the first inner radius, and the upper flange portion of theshell has third inner radius which is substantially the same as thesecond inner radius of the head flange portion and smaller than thefirst inner radius of the first shell portion.